You can easily find out where a cost center GROUP is used - there's a menu choice for it on the display of the group. The method Kevin proposes will unfortunately not work unless the element you want is the low or high value in the group, or if you're looking for a range. It will not find something in the middle of a range.

There is a function module in SAP, "K_GROUP_REMOTE_READ". Using the setclass 0102 and values from the SETNODE table, a program could be written to figure out whether a cost element is being used in the various groups. You will need an ABAP programmer to do this, however.